We plan to explore further the ability of postthymic T cells to repopulate the various functionally distinct T-cell compartments in thymectomized, irradiated and bone marrow-supplemented syngeneic recipient mice. Our initial studies have suggested that there are at least two lineages of these postthymic T-cell "progenitors," one of which gives rise to cytotoxic T cells (CTL) and the other to IL-2-producing helper cells. Our approach will be to allow selected T-cell subsets to repopulate T-cell depleted mice. Three months after reconstitution mice will be tested, using both conventional and limiting dilution methods, to see whether they have regenerated antigen-responsive (and mitogen-responsive) cells of the helper, proliferating and cytotoxic lineages. We hope to answer several questions by studying mice reconstituted with different postthymic T-cell populations: How many progenitors are there of each lineage? How many mature, antigen-reactive cells can be produced from each progenitor cell, and how rapidly? Can the progenitors of each lineage be distinguished by surface antigen from one another, and from the mature cells to which they give rise? Do the cells produced by postthymic maturation differ from those present in normal mice? And lastly, can this developmental sequence be influenced by specific antigen, thymic hormones, or interleukins (1 or 2)?